1. Field of the Invention
The present invention relates to an objective lens for use in an optical head device for recording and reproducing data with respect to optical recording mediums such as a CD and a DVD having different thicknesses of transparent substrates, using laser light having different wavelengths. The present invention also relates to an optical head device provided with the objective lens.
2. Description of the Related Art
An optical head device has heretofore been known which records and reproduces data with respect to optical recording mediums having different thicknesses of transparent substrates for protecting recording surfaces, and different recording densities, using laser light having different wavelengths. Examples of the optical recording medium include a CD, a DVD and the like. Here, in the CD, the thickness of the transparent substrate which protects the recording surface is 1.2 mm. In the DVD, the thickness of the transparent substrate is 0.6 mm, and is smaller than that of the CD, and a recording density is higher than that of the CD. Therefore, for example, the laser light having a wavelength of 790 nm is used in the recording/reproducing with respect to the CD, whereas laser light having a wavelength of 660 nm is used in the recording/reproducing with respect to the DVD. In the objective lens of the optical head device, a numerical aperture NA1 required in the recording/reproducing with respect to the CD is, for example, 0.45, and an numerical aperture NA2 required in the recording/reproducing of the DVD is, for example, 0.6.
In this type of optical head device, in order to achieve miniaturization and thickness reduction of the device, an arrangement has been proposed in which laser light having a wavelength corresponding to each optical recording medium is condensed on the recording surface of the optical recording medium using a single optical condensing system (see, e.g., Japanese Patent Application Laid-Open No. 10-55564). In the optical head device described in Japanese Patent Application Laid-Open No. 10-55564, an objective lens is used which has a refractive surface comprising a middle region centering on an optical axis and a peripheral region concentrically formed on an outer peripheral side of the middle region.
As described above, for example, since the numerical aperture NA1 required in the recording/reproducing with respect to the CD is 0.45, and a numerical aperture NA2 required in the recording/reproducing with respect to the DVD is 0.6, the laser light in the region outside the numerical aperture NA1 is a noise component which does not contribute the recording/reproducing in the recording/reproducing with respect to the CD in a case where the single optical condensing system is used. Therefore, since the vicinity of the numerical aperture NA1 required in the recording/reproducing with respect to the CD is a boundary between the middle region and the peripheral region in the refractive surface of the objective lens, this boundary portion is provided with a discontinuous surface whose spherical aberration is discontinuous is formed. In this arrangement, at a recording/reproducing time of the CD, the laser light of the region outside NA1 is prevented from being condensed on the recording surface of the CD, and noises are prevented from being generated.